Premix burner with firing rate control

ABSTRACT

An apparatus for heating a process chamber. The apparatus includes a furnace structure that defines a reaction zone that communicates with the process chamber. A burner structure defines a burner port and communicates with the reaction zone through the burner port. The burner structure includes a plurality of mixer tubes. The mixer tubes mix fuel and oxidant and define flow paths for the fuel and oxidant. Each of the flow paths extends through a respective mixer tube. A valve assembly interrupts a flow of fuel and oxidant along one of the flow paths. The valve assembly is shiftable between a closed condition interrupting a flow of fuel and oxidant along the flow path, and an open condition not interrupting the flow. A controller operates to shift the valve assembly into or out of its open condition. The controller has a mode of operation including consecutive cycles in which the valve assembly is shifted into and out of its open condition.

[0001] This application is a Continuation-In-Part of U.S. Pat. No. 6,312,250 entitled “Premix Burner with Firing Rate Control,” filed Mar. 9, 2000 that claims priority from provisional patent application Serial No. 60/130,006, filed Apr. 19, 1999.

FIELD OF THE INVENTION

[0002] The present invention relates to a burner apparatus having a reaction zone in which a premix of fuel and oxidant is ignited and undergoes combustion to heat a process chamber communicating with the reaction zone.

BACKGROUND OF THE INVENTION

[0003] A premix burner is part of an industrial furnace having a process chamber in which a drying or heating process is performed. The burner has a reaction zone communicating with the process chamber. A mixture of fuel and oxidant, which is known as a premix, is ignited and burned in the reaction zone to provide thermal energy for heating the process chamber. The premix is formed upon intermixing of the fuel and oxidant along flow paths that convey the fuel and oxidant to the reaction zone.

[0004] The combustion conditions in the reaction zone can be controlled by controlling the firing rate at which the premix is ignited upon entering the reaction zone. The firing rate is generally controlled by modulating the velocity at which the premix enters the reaction zone. The velocity is modulated uniformly throughout all of the premix flow paths leading to the reaction zone.

[0005] Modulating the premix flow velocity has certain limitations as a way to control the firing rate of the burner. First, the practical velocity turn-down range is limited by flashback. Flashback occurs when premix flow velocity decreases sufficiently to allow flame to propagate upstream along the flow paths leading to the reaction zone. Second, ultra low NO_(x) emissions, and to some extent very low CO emissions, depend on excellent mixing of the fuel and oxidant forming the premix. Unfortunately, mixing quality can sometimes deteriorate as the flow path velocity and pressure drop decrease when the burner is turned down in a conventional manner.

[0006] Additionally, premix burners can amplify or cancel noise, depending in part on the velocity at which the premix flows toward and into the reaction chamber. The burner can be tuned for noise accordingly, but conventional turn-down changes the premix velocity and thus changes the noise tuning of the burner. This limits the velocity turn-down range for some noise-prone applications. The minimum velocity may thus be limited by flashback, emissions levels, and noise tuning limits.

[0007] Increasing the maximum velocity in a premix burner is one way to increase the turn-down range. Increasing the maximum velocity and reducing the size of the burner increases the turn-down range by increasing the amount of turn-up. However, increasing the turn-down range with a higher maximum velocity can significantly increase pressure requirements and, therefore, power costs. Accordingly, increasing the maximum premix flow velocity can be an expensive way to increase the turn-down range.

[0008] Conventional control of the burner firing rate can also be rather slow. The transition from a low to a high firing rate may take from thirty seconds to several minutes, depending on the speed of the fuel and oxidant control devices, and also on the ability of the ratio control system to maintain the fuel to oxidant ratio. Many low NO_(x) burners require precise ratio control that can be maintained adequately only when the firing rate is changed slowly. This might not be suitable for applications that require a rapid firing rate response for optimum performance.

SUMMARY

[0009] The present invention provides an apparatus for heating a process chamber. The apparatus includes a furnace structure that defines a reaction zone that communicates with the process chamber. A burner structure defines a burner port and communicates with the reaction zone through the burner port. The burner structure includes a plurality of mixer tubes. The mixer tubes mix fuel and oxidant and define flow paths for the fuel and oxidant. Each of the flow paths extends through a respective mixer tube. A valve assembly interrupts a flow of fuel and oxidant along one of the flow paths. The valve assembly is shiftable between a closed condition interrupting a flow of fuel and oxidant along the flow path, and an open condition not interrupting the flow. A controller shifts the valve assembly into or out of its closed condition. The controller has a mode of operation including consecutive cycles in which the valve assembly is shifted into and out of its open condition.

[0010] In accordance with another feature of the invention, a premix burner apparatus for combusting premix in a reaction zone is provided. The burner apparatus includes a burner structure defining first and second oxidant plenums. A first mixer tube communicates with the first oxidant plenum and a second mixer tube communicates with the second oxidant plenum. The burner apparatus also includes an oxidant supply system that includes first and second oxidant sources. The first oxidant source communicates with the first oxidant plenum, and the second oxidant source communicates with the second oxidant plenum. A control system controls flows of oxidant from the oxidant sources to the oxidant plenums, and interrupts a first one of the oxidant flows while continuing a second one of the oxidant flows. The control system includes valve assemblies that are shiftable between a closed condition interrupting the first one of the oxidant flows and an open condition not interrupting the first one of the oxidant flows. The control system further includes a controller that shifts the valve assemblies into and out of their open conditions. The controller has a mode of operation that includes consecutive cycles in which the valve assemblies are shifted into or out of their closed conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic view of parts of an apparatus comprising a first embodiment of the claimed invention;

[0012]FIG. 2 is an enlarged partial view of parts shown in FIG. 1;

[0013]FIGS. 3 and 4 are views similar to FIG. 2 showing parts in different positions;

[0014]FIG. 5 is a partial view of an apparatus comprising a second embodiment of the invention;

[0015]FIGS. 6 and 7 are graphs of performance characteristics of the first embodiment of the invention;

[0016]FIG. 8 is a schematic view of an apparatus comprising a third embodiment of the invention;

[0017]FIG. 9 is a schematic view of an apparatus comprising a fourth embodiment of the invention;

[0018]FIGS. 10 and 11 are graphs of performance characteristics of the first embodiment of the invention;

[0019]FIG. 12 is a schematic view of additional parts of the first embodiment of the invention;

[0020]FIG. 13 is a graph of performance characteristics of the first embodiment of the invention;

[0021]FIG. 14 is a view similar to FIG. 12 showing parts of an apparatus comprising a fifth embodiment of the invention;

[0022]FIG. 15 is a graph of performance characteristics of the fifth embodiment of the invention;

[0023]FIG. 16 is a schematic view of an apparatus comprising sixth embodiment of the invention; and

[0024]FIG. 17 is a schematic view of an apparatus comprising a seventh embodiment of the invention.

DESCRIPTION

[0025] A first embodiment of the invention is shown schematically in FIG. 1. The first embodiment includes a burner 10, which is part of an industrial furnace having a process chamber 12. A drying or other heating process is performed on a load (not shown) in the chamber 12. Thermal energy for the heating process is generated in a reaction zone 14 in the burner 10. This occurs upon combustion of a fuel and oxidant mixture in the reaction zone 14. Specifically, the burner 10 of FIG. 1 is a premix burner in which fuel from a fuel source 16 is mixed with oxidant from an oxidant source 18 to form a premix. The premix undergoes combustion in the reaction zone 14 to provide thermal energy to the adjoining process chamber 12.

[0026] The parts of the burner 10 that are shown in FIG. 1 include a housing structure 20 and a plurality of valve assemblies 22. An oxidant supply plenum 24 is defined within the housing structure 20. Six mixer tubes 26, two of which are shown in FIG. 1, are arranged in the plenum 24 in a cylindrical array centered on an axis 27. The inner ends 28 of the mixer tubes 26 are located within the plenum 24. The outer ends 30 of the mixer tubes 26 define respective entrances to the reaction zone 14.

[0027] The valve assemblies 22 are operative to provide controlled flows of fuel and oxidant along the mixer tubes 26. All of the valve assemblies 22 in this embodiment are located within the plenum 24 so as to receive a common supply of oxidant from the oxidant source 18. Each valve assembly 22 receives a supply of fuel from the fuel source 16 separately from each other valve assembly 22. Moreover, each valve assembly 22 is operatively engaged with the inner end 28 of a single corresponding mixer tube 26. In this arrangement, each valve assembly 22 is operative to provide the corresponding mixer tube 26 with controlled flows of fuel and oxidant separately from the controlled flows of fuel and oxidant in any other mixer tube 26.

[0028] As shown in FIG. 1, the valve assemblies 22 are included in a firing rate control system 40 with a controller 42 and a corresponding plurality of actuators 44. The valve assemblies 22 in the first embodiment of the invention, as well as the actuators 44, are alike and have the configuration shown by way of example in FIG. 2. The mixer tubes 26 in the first embodiment also are alike. Each has a cylindrical configuration with a longitudinal central axis 49, as shown partially in FIG. 2. The inner end 28 of the mixer tube 26 is preferably flared radially outward in a generally bell-shaped configuration.

[0029] The valve assembly 22 includes a first movable valve plate 50, a second movable valve plate 52, and a control rod 54. The actuator 44 moves the control rod 54 back and forth along the axis 49. The control rod 54 interacts with the valve plates 50 and 52 to move them back and forth between the positions in which they are shown in FIGS. 2, 3 and 4. Each actuator 44 is thus operative to shift the corresponding valve assembly 22 throughout a range of conditions. These include a fully closed condition, as shown in FIG. 2, a partially open condition, as shown for example in FIG. 3, and a fully open condition, as shown in FIG. 4.

[0030] The control rod 54 extends closely through an aperture 55 at the center of the first valve plate 50. The aperture 55 permits sliding movement of the first valve plate 50 axially between a pair of stop members 56 and 58 that are fixed to the rod 54. A first spring 60 is compressed axially between the first valve plate 50 and the first stop member 56.

[0031] The second valve plate 52 is located within a housing portion 62 of the valve assembly 22. The housing 62 defines a fuel supply plenum 64 with an inlet 65 for receiving fuel from the fuel source 16 (FIG. 1). Three fuel injector tubes 66, two of which are shown in FIG. 2, project from the housing 62 into the mixer tube 26 through apertures 67 in the first valve plate 50. The control rod 54 extends through a pair of apertures 69 in the housing 62, and also through an aperture 71 at the center of the second valve plate 52.

[0032] When the valve assembly 22 is in the fully closed condition of FIG. 2, the first valve plate 50 abuts the inner end 28 of the mixer tube 26 to block the flow of oxidant from the oxidant supply plenum 24 into the mixer tube 26. The first spring 60 holds the first valve plate 50 firmly in that position. A second spring 72 holds the second valve plate 52 firmly against an inner housing structure 74. The second valve plate 52 then closes an opening 75 (best shown in FIG. 4) in the inner housing structure 74 to block the flow of fuel from the fuel supply plenum 64 to the injector tubes 66 through the opening 75.

[0033] The control rod 54 is moved axially from right to left, as viewed in the drawings, when the valve assembly 22 is shifted from the fully closed condition of FIG. 2 to the partially open condition of FIG. 3. The second stop member 58 at the end of the control rod 54 moves against the first valve plate 50 so as to draw the first valve plate 50 axially away from the end 28 of the mixer tube 26. This enables oxidant from the oxidant supply plenum 24 to flow into the mixer tube 26. However, the second valve plate 52 remains in its closed position while the control rod 54 slides axially through the aperture 71 until an opener 80 on the rod 54 moves against the second valve plate 52. The opener 80 then draws the second valve plate 52 axially away from the opening 75 against the bias of the second spring 72 upon movement of the control rod 54 fully to the position in which it is shown in FIG. 4. Fuel can then flow from the fuel supply plenum 64 to the injector tubes 66 through the opening 75, and further from the injector tubes 66 into the mixer tube 26 through nozzles 82 at the ends of the injector tubes 66.

[0034] The first and second valve plates 50 and 52 in the first embodiment of the invention are linked together such that the partially open condition of FIG. 3 is interposed between the fully closed condition of FIG. 2 and the fully open condition of FIG. 4. This is a safety feature which ensures that the second valve plate 52 can be shifted from its closed position to an open position, and thereby to allow fuel to flow into the mixer tube 26, only when the first valve plate 50 is in an open position allowing oxidant also to flow into the mixer tube 26.

[0035] The burner 10 (FIG. 1) has a preferred mode of operation in which oxidant flows from the oxidant source 18 equally to all of the valve assemblies 22, and fuel flows from the fuel source 16 equally to all of the valve assemblies 22. The pressure of the oxidant flowing from the source 18 to the plenum 24 is controlled, as is the pressure of the fuel flowing from the source 16 to the plenums 64 (FIG. 2). Accordingly, when the valve assemblies 22 are all in their fully open conditions, a premix is formed in the mixer tubes 26, and the fuel to oxidant ratio of the premix is maintained at a substantially constant value corresponding to the pressures of the fuel and oxidant supplied to the plenums 24 and 64. The velocity of the premix emerging from the mixer tubes 26 also has a substantially constant value. The firing rate at the entrances 30 to the reaction zone 14 is likewise maintained at a substantially constant value as long as all of the valve assemblies 22 remain in their fully open conditions. However, in accordance with the invention, the control system 40 can vary the firing rate at the entrances 30 without substantially varying the velocity at which the premix enters the reaction zone 14.

[0036] The controller 42 (FIG. 1) operates the actuators 44 so as to shift the valve assemblies 22 between their open and closed conditions, and thereby to turn the corresponding mixer tubes 26 ON and OFF, for control of the burner firing rate in accordance with the invention. In the first embodiment of the invention, the controller 42 has a plurality of differing modes of operation. A first mode of operation comprises consecutive cycles in which a single valve assembly 22 is shifted back and forth between its fully open and fully closed conditions while the remainder of the valve assemblies 22 remain in their fully open conditions. Such shifting of a valve assembly 22 causes corresponding interruptions of the premix flow in the adjoining mixer tube 26. The same valve assembly 22 can be shifted in each cycle, but it may be desirable to shift a different one of the valve assemblies 22 in each cycle in order to prolong the working life of the actuators 44 and valve assemblies 22. Increasing the durations of the cycles also helps to prolong the working life of the actuators 44 and valve assemblies 22.

[0037] This method of on-off cycling varies the firing rate at the reaction zone entrances 30 by providing the entrances 30 with an effective premix flow area that differs from their total flow area potential. The effective premix flow area is equal to the average, over time, of the differing total flow areas that are utilized upon intermittent reduction of the number of mixer tubes 26 carrying premix to the entrances 30. This enables precise control of the firing rate because the effective premix flow area can have a fractional value that differs from the sum of any whole number of individual entrance flow areas.

[0038] Another mode of operation comprises shifting a selected group of valve assemblies 22 simultaneously. This mode comprises shifting a selected group of valve assemblies 22 into or out of their fully closed conditions, and subsequently back to their previous conditions, while the remainder of the valve assemblies 22 remain in their fully open or closed conditions. The same or a different group of valve assemblies 22 can be shifted in consecutive cycles. A group of valve assemblies 22, or all of the valve assemblies 22, can likewise be shifted sequentially rather than simultaneously. Other modes of operation could differ in other ways, such as in the frequency or duration of cycles. In each case, the flows of premix emerging from any one or more of the mixer tubes 26 can be interrupted independently from each other, with each interruption providing a corresponding reduction in the firing rate of the burner 10.

[0039] On-off cycling of the mixer tubes 26 can be especially effective for combustion applications in which large thermal masses are heated in the process chamber 12. A large thermal mass may have a correspondingly long thermal time constant. Such a mass will be heated uniformly over time if its thermal time constant is long compared to the on-off cycle times. A smaller thermal mass also can be heated uniformly over time if the on-off cycle times are appropriately short.

[0040] As noted above, the mixer tubes 26 in the first embodiment of the invention are alike. As shown schematically in FIG. 5, a second embodiment of the invention includes differently sized mixer tubes 100, 102 and 104, with respective entrances 106, 108 and 110 to a reaction zone 112. These mixer tubes 100, 102 and 104 preferably have bell-shaped inner ends like the inner ends 28 described above, but otherwise have uniform diameters. When the flows of premix in the smallest mixer tubes 100 are cycled ON and OFF in accordance with the invention, the cycle to cycle variations in firing rate are smaller than those that occur upon corresponding on-off cycling at the larger mixer tubes 102 and/or 104. More precise control of the firing rate is possible with this embodiment of the invention because the differing sizes of the mixer tubes 100, 102 and 104 enable a greater number of effective premix flow area combinations to be achieved upon on-off cycling at the various mixer tubes 100, 102 and 104, by comparison to a plurality of mixer tubes of the same size.

[0041] In addition to on-off cycling at the mixer tubes, the invention can be used for turndown and turn-up of a burner without substantially varying the velocity at which the premix enters the reaction zone. This is indicated in FIGS. 6 and 7 with reference to a burner having six mixer tubes like the mixer tubes 26 in the first embodiment. In both figures the heavy black dots represent the operating conditions of the burner under an on-off control regime, while the dotted lines represent the operating conditions of the burner with on-off control of mixers as well as modulating control of the whole burner. In FIG. 6, turn-down of the burner firing rate is achieved each time a mixer tube is turned off, and turn-up is achieved each time a mixer tube is turned on. With only on-off control, this leads to only discrete points of firing rate that can be achieved. With the addition of modulating control, the other firing rates in between the discrete points can be achieved.

[0042]FIG. 7 shows a plot of mixer velocity versus burner firing rate. Under on-off control, the mixer velocity changes as mixers are turned on and off dependent on the characteristics of the oxidant supply system. In this example, the velocity increases as mixers are turned off and decreases as mixers are turned on, but only discrete values of firing rate and velocity are obtained. When modulating control is added, the velocity profile follows the dashed lines. As modulation is used to reduce the burner firing rate, the velocity decreases linearly, and when a mixer is turned off, the velocity increases at that firing rate.

[0043] The velocity can be held within a relatively narrow velocity band throughout most or all of the wide burner turn-down range because the turn-down is achieved by turning mixers OFF rather than by turning the velocity down. This can avoid changes in noise characteristics of the burner at different velocities. This can also curb the high-end pressure requirement of the burner at high velocities and avoid the propensity to flash back at low velocities.

[0044] Smoother and more continuous turn-down can further be achieved by using multiple mixer sizes. One or more smaller on-off mixers can be used in conjunction with one or more larger mixers, some or all of which may have on-off control. The smaller on-off mixers provide small turn-down steps to smooth the gaps between the larger steps of the larger on-off mixers. For example, six large mixers might each provide fifteen percent of the total burner input, and two smaller mixers might each provide only five percent of the total burner input. The burner could be turned up and down in five percent on-off steps through proper on-off switching of the mixers to meet the heat demand of the burner. This would provide relatively fine control of heat input for most applications. Combined with mixer cycling, any practical heating requirement can be met smoothly.

[0045] Even finer turn-down control can be obtained by using a wider range of smaller mixers. For example, in a variation of the second embodiment shown in FIG. 5, firing rate control in one percent increments can be achieved with nine mixers having the following size percentages: 1, 2, 2, 5, 10, 20, 20, 20, and 20.

[0046] In accordance with an additional feature of the invention, the speed of the on-off control at a mixer tube 26 is limited only by the speed at which the corresponding control rod 54 is moved by the actuator 44, which preferably comprises a fast acting solenoid or the like. Fast on-off control at each mixer tube 26 is a valuable characteristic for some processes that experience sudden changes in heat requirements. The fast on-off control may be combined with a supplementary burner system (not shown). Such a supplementary burner system would preferably comprise the apparatus described in copending U.S. Patent Application Serial No. 60/126,472, filed Mar. 26, 1999, entitled A Premix Burner with Integral Mixers and Supplementary Burner System, which is incorporated herein by reference. A single fuel to oxidant ratio control system can be used to control fuel pressure and flow to the mixer tubes 26 and the supplementary burner system. In some cases it may be possible to operate the burner 10 at a low firing rate with only the supplementary burner system to maintain heat in the reaction zone 14, and then to shift to a higher firing rate in a few seconds by quickly shifting valve assemblies 22 open to turn the corresponding mixer tubes 26 on. The demand for a full firing rate can trigger a signal to the controller 42 to turn all or most of the mixer tubes 26 on simultaneously, or nearly simultaneously, in this manner. The burner 10 would then reach full input without any significant interruption of ignition because the supplementary burner system would anchor ignition and prevent any build up of unburned premix in the reaction zone 14 that could ignite in an undesirable way.

[0047] A burner 200 comprising a third embodiment of the invention is shown partially in the schematic view of FIG. 8. The burner 200 has mixer tubes 202 with entrances 204 to a reaction zone 206. The burner 200 further has a housing structure 208 defining an oxidant supply plenum 210 from which the mixer tubes 202 receive oxidant at their inner ends 212. Fuel is injected into the mixer tubes 202 by fuel injectors 214 that are located downstream of the inner ends 212.

[0048] Like the burner 10 described above, the burner 200 further has a control system 220 which is operative to control flows of fuel and oxidant along the mixer tubes 202 separately from each other. The control system 220 includes an actuator 222 and a valve member 224 for each mixer tube 202. The actuators 222 operate separately under the direction of a controller 226 to shift the valve members 224 into and out of closed positions in which they block flows of oxidant from the plenum 210 into the mixer tubes 202. A plurality of fuel control valve assemblies 228, and a corresponding plurality of actuators 230, also operate under the influence of the controller 226 in accordance with the invention, but are separate from the valve members 224. The control system 220 is otherwise operable to control the firing rate at the reaction zone entrances 204 by opening and closing the valve assemblies 224 and 228 in substantially the same manner as described above with reference to the control system 40. Additionally, the control system 220 is further operative to modulate the premix velocity and the fuel to oxidant ratio at each entrance 204, separately from each other entrance 204, by separately shifting the fuel control valve assemblies 228 throughout ranges of differing open conditions.

[0049] A fourth embodiment of the invention also uses modulating control in addition to on-off control in accordance with the invention. As shown in FIG. 9, the fourth embodiment is a burner 300 which includes many parts that are substantially the same as corresponding parts of the burner 10 described above. This is indicated by the use of the same reference numbers for such corresponding parts in FIGS. 1 and 9. However, the burner 300 includes alternative valve assemblies 302 in place of the valve assemblies 22 described above. Like each valve assembly 22, each valve assembly 302 is shiftable between open and closed conditions for on-off control of the corresponding mixer tube 26. Each valve assembly 302 is further shiftable between a range of intermediate conditions for modulating control of the mixer tube 26. The range of intermediate conditions of each valve assembly 22 provides a corresponding range of ratios at which fuel from the fuel source 16 and oxidant from the oxidant source 18 are together admitted to form a premix in the mixer tube 26. The actuators 44 are operative to shift the valve assemblies 302 separately from each other under the influence of the controller 42 so that the control system 40 provides a wide range of firing rate control at the reaction zone entrances 30 in accordance with the invention.

[0050] Additional on-off performance characteristics of the invention are shown in FIGS. 10 and 11. In the first embodiment of the invention described above, the oxidant source 18 (FIG. 1) is an air blower of known construction. The curve 400 of FIG. 10 is the blower curve. This curve 400 represents values of outlet pressure and flow rate for the particular blower 18. The curve 402 directly beneath the blower curve 400 is the air supply curve. This curve 402 represents values of pressure and air flow at the outlet 403 (FIG. 1) of the air supply system which communicates the blower 18 with the burner 10. The air supply curve 402 differs from the blower curve 400 because of resistance in the air supply system. This example of an air supply curve represents a particular constant value of that resistance.

[0051] The curve 404 of FIG. 10 is a burner resistance curve. This curve 404 represents values of pressure and air flow at the burner air inlet 405 (FIG. 1) when all of the six mixer tubes 26 are ON. A second burner resistance curve 406 represents values of pressure and air flow at the burner air inlet 405 when only one of the six mixer tubes 26 is ON. The second burner resistance curve 406 differs from the first burner resistance curve 404 because of the greater resistance to a given flow of air through the burner 10 along only a single mixer tube 26. Accordingly, the point 410 where the first burner resistance curve 404 intersects the air supply curve 402 represents the values of pressure and air flow at the burner air inlet 405 when all of the six mixer tubes 26 are ON. The point 412 where the second burner resistance curve 406 intersects the air supply curve 402 represents the values of pressure and air flow at the burner air inlet 405 when only one of the six mixer tubes 26 is ON.

[0052] In a similar manner, the curves of FIG. 11 represent values of pressure and flow for the fuel supplied to the mixer tubes 26 in the burner 10. The source 16 of fuel in the first embodiment of the invention is a utility supply of natural gas. As described below with reference to FIG. 12, a fuel supply system 500 conveys the gas from the source 16 to the burner 10. The fuel supply system 500 includes a pressure reducing regulator 502 which, as known in the art, provides and maintains a constant output pressure within a range of selectable output pressures. The horizontal line 504 of FIG. 11 represents the output pressure at the regulator 502. The curve 506 of FIG. 11 is a fuel supply curve. This curve 506 represents values of pressure and fuel flow at the outlet 507 of the fuel supply system 500. The fuel supply curve 506 differs from the regulator output line 504 because of resistance in the fuel supply system 500 between the regulator 502 and the burner 10. As with the air supply curve 402, this example of a fuel supply curve represents a particular constant value of resistance.

[0053] The curve 508 of FIG. 11 is a burner resistance curve. This curve 508 represents values of pressure and fuel flow at the burner fuel inlet 509 when all of the six mixer tubes 26 are ON. A second burner resistance curve 510 of FIG. 11 represents values of pressure and fuel flow at the burner fuel inlet 509 when only one of the six mixer tubes 26 is ON, and differs from the first burner resistance curve 508 because of the greater resistance to a given flow of fuel through the burner 10 along only a single mixer tube 26. The point 512 where the first burner resistance curve 508 intersects the fuel supply curve 506 represents the pressure and fuel flow at the burner fuel inlet 509 when all of the six mixer tubes 26 are ON. The graphs of FIGS. 10 and 11 are scaled such that the flow of fuel at the intersection point 512 (FIG. 11), as a relative percentage, coincides with the flow of air at the intersection point 410 (FIG. 10). This indicates that the burner 10 is operating at a specified fuel to oxidant ratio.

[0054] The second intersection point 514 of FIG. 11 represents values of pressure and fuel flow at the burner fuel inlet 509 when only one of the six mixer tubes 26 is ON. The second intersection point 514 of FIG. 11 does not coincide with the second intersection point 412 of FIG. 10. This is because the fuel supply curve 506 extends between the burner resistance curves 508 and 510 with a curvature that, because of inherent differences in device characteristics, differs from the curvature of the air supply curve 402 between the corresponding burner resistance curves 404 and 406. Accordingly, when all but one of the six mixer tubes 26 are turned OFF, the fuel flow decreases differently from the air flow. This changes the fuel to oxidant ratio. Such disruption of the fuel to oxidant ratio can be reduced by appropriate operation and control of the fuel supply system 500.

[0055] Referring more specifically to FIG. 12, the fuel supply system 500 has two distinct portions 520 and 522 between the fuel source 16 and the burner 10. The first portion 520 of the fuel supply system 500 is a supervisory portion which includes at least a pair of supervisory shut-off valves 524 in series with the pressure reducing regulator 502. The second portion 522 of the fuel supply system 500 is a metering and flow control portion. That portion 522 of the fuel supply system 500 includes a flow measuring device 526 and a motorized control valve 528.

[0056] A controller 530 monitors the flow rate indicated by the measuring device 526, and compares it with a corresponding flow rate in the air supply system (not shown). A comparison of those flow rates may indicate a deviation from the specified fuel to oxidant ratio. If so, the controller 530 shifts the control valve 528, and may also shift a counterpart control valve in the air supply system, to direct the fuel and oxidant back toward the specified ratio.

[0057] When the controller 530 shifts the control valve 528 in the foregoing manner, it varies the flow resistance of the fuel supply system 500. This changes the fuel supply curve 506 of FIG. 11. The controller 530 thus provides a new fuel supply curve such as, for example, the fuel supply curve 540 of FIG. 13. As compared with the previous fuel supply curve 506, the new fuel supply curve 540 intersects the second burner resistance curve 510 at a point 542 that coincides with its counterpart 412 in FIG. 10. This indicates that the burner 10 will again operate at the specified fuel to oxidant ratio although five of the six mixer tubes 26 have been turned off. However, when a fuel supply curve is changed upon shifting of the control valve 528, the constant output pressure of the regulator 502 constrains the curve to move only pivotally about the point 543 where the curve diverges from the regulator supply line 504. This causes the new fuel supply curve 540 to intersect the first burner resistance curve 508 at a point 544 that is spaced greatly from the original point 512 of intersection with that curve 508. Therefore, when the five mixer tubes 26 are turned back ON so that the burner 10 once again has all six mixer tubes 26 ON, the fuel flow at the new intersection point 544 will differ greatly from the fuel flow at the original intersection point 512. The ratio of fuel to oxidant will likewise differ from the specified ratio. This problem is avoided by operation of the alternative fuel supply system 600 of FIG. 14.

[0058] The fuel supply system 600 has a supervisory portion 602 that includes a pressure reducing regulator 604 and redundant supervisory shut-off valves 606. Those parts 604 and 606 of the fuel supply system 600 are substantially the same as the corresponding parts of the fuel supply system 500. The fuel supply system 600 further has a metering and flow control portion 608 that differs from the corresponding portion 522 of the fuel supply system 500. Specifically, the fuel supply system 600 includes a flow measuring device 610 and a motorized control valve 612, and further includes a pressure regulating device such as, for example, a pressure regulator 614. Unlike the regulator 604, the regulator 614 is equipped with an actuator 616, which is operated by a controller 618. The regulator 614 and the control valve 612 operate in series to change the fuel supply curve differently from the manner in which the control valve 528 changes the fuel supply curve. This is indicated in FIG. 15, which shows a new fuel supply curve 620 that can be obtained by use of the regulator 614 and the control valve 612 in accordance with the invention.

[0059] The transition from the original fuel supply curve 506 to the new fuel supply curve 620 is accomplished in two phases. In one phase of transition, the controller 618 directs the actuator 616 to decrease the output pressure of the regulator 614. This causes the curve 506 to translate uniformly downward toward the horizontal axis of FIG. 15, and thereby to move to a location at which it intersects the second burner resistance curve 510 at a point 622 that coincides with the point 542 of FIG. 13. This ensures that the burner 10 will operate at the specified fuel to oxidant ratio when only one of the six mixer tubes 26 is ON. In the other phase of transition, the control valve 612 is shifted so as to vary the resistance between the regulator 614 and the burner 10, and thereby to move the curve 506 pivotally until it intersects the first burner resistance curve 508 at a point 624 that coincides with the point 512 of FIG. 13. This ensures that the burner 10 will operate at the specified fuel to oxidant ratio when all six of the mixer tubes 26 are ON. It may be necessary to perform these phases of curve transition in iterations, either sequentially or simultaneously, until satisfactory intersection points are reached. In each case, the regulator 614 and the control valve 612 are shifted until the controller 618 determines that the flow conditions indicated by the curves of FIG. 15 include the specified fuel to oxidant ratio when the mixer tubes 26 are turned OFF and ON in accordance with the present invention.

[0060] As described above, the fuel supply system 600 enables a fuel supply curve to translate vertically as well as to pivot, and thus enables a greater degree of equality to be achieved for the curvatures of a fuel supply curve and an oxidant supply curve. This enables the ratio of fuel to oxidant to be maintained with a correspondingly greater degree of precision for on-off control of a burner. Moreover, when an appropriate fuel supply curve has been established by practicing this feature of the invention, further iterations of curve transition may not be necessary to maintain a specified fuel to oxidant ratio during subsequent on-off control of the burner. It may thus be preferable for the motorized control valve 612 of FIG. 14, which shifts under the influence of the controller 618, to be replaced with a manually shiftable control valve. The manually shiftable control valve could be shifted to a condition in which the applied resistance imparts an appropriate pivotal orientation to the fuel supply curve, and could thereafter be allowed to remain in that condition.

[0061] A differently configured furnace may be used in accordance with the invention. An apparatus 700 comprising a sixth embodiment of the invention is shown in FIG. 16. The apparatus 700 is a furnace structure that has parts that are substantially the same as corresponding parts of the burner 10. For instance, the process chamber 12 and the valve assemblies 22 are the same. However, the furnace structure 700 includes a burner structure 702 and a fuel structure 720 that differ from those in the previous embodiments.

[0062] The burner 702 has inner surfaces spaced radially from the axis 27 to define the sides of a plenum 710. A perforated plate 730 is centered on the axis 27 and defines a first end of the plenum 710. An array of mixer tubes 736 is also centered on the axis 27 and is spaced axially from the plate 730 to define a second end of the plenum 710.

[0063] The plate 730 has annular inner surfaces that define a plurality of burner ports 732. The plenum 710 communicates with the reaction zone 14 through the array of burner ports 732. The mixer tubes 736 define respective flow paths 738 for the fuel and oxidant to flow from the valve assemblies 22 to the plenum 710.

[0064] The fuel structure 720 allows the fuel source 16 to communicate with the reaction zone 14 through fuel ports 724. The fuel ports 724 are oriented to direct fuel to flow into the reaction zone 14 in directions indicated by arrows 742.

[0065] A control system 750 includes a controller 752, which is similar to the controllers 42, 226, 530 and 618 described above. The control system 750 also includes a plurality of valves 756 and the valve assemblies 22, which are controlled by the controller 752. Some of the valves 756 are located between the fuel source 16 and the fuel ports 724. Others are located between the fuel source 16 and the valve assemblies 22, as well as between the oxidant source 18 and the valve assemblies 22.

[0066] The valves 756 can have open, closed and partially open conditions. Respective flows of fuel and oxidant are interrupted if the valves 756 are in a closed condition and the respective flows are not interrupted if the valves 756 are in an open condition. The valves 756 restrict the respective flows if they are in the partially open condition. Further, the partially open condition includes a plurality of valve settings of differing degrees of restriction.

[0067] The controller 752 can operate the burner 702 in different modes. In particular, the controller 752 can operate in a low temperature or a high temperature mode and can switch between them. The controller 752 also has a mode of operation in which different valve assemblies 22 can be shifted into or out of their closed conditions in consecutive cycles. Predetermined criteria, such as time or reaction zone temperature, can be used to trigger the controller 752 to switch from a mode of operation to a different mode of operation.

[0068] In the low temperature mode of operation, the controller 752 can supply fuel and oxidant to the valve assemblies 22, and further to the plenum 710, at a respective first flow rate. Specifically, the controller 752 can control some of the valves 756 to switch to a partially open condition that would restrict fuel and oxidant to flow from the fuel and oxidant sources 16 and 18 to the valve assemblies 22 at the first flow rate.

[0069] In the high temperature mode of operation, the controller 752 can supply fuel and oxidant to the valve assemblies 22, and further to the plenum 710, at a respective second, higher flow rate. Specifically, the controller 752 can control some of the valves 756 to switch to a partially open condition that would restrict fuel and oxidant to flow from the fuel and oxidant sources 16 and 18 to the valve assemblies 22 at the second, higher flow rate.

[0070] The fuel and oxidant can be directed along the flow paths 738 by the mixer tubes 736 and into the plenum 710. Flows of fuel and oxidant are indicated by directional arrows 740. The flows of fuel and oxidant 740 can commingle in the plenum 710. The commingled fuel and oxidant can then directed by the plenum 710 out through the array of burner ports 732 and into the reaction zone 14.

[0071] The controller 752 can start operating by controlling some of the valves 756 to switch from their closed to their open condition. With some of the valves 756 switched to the open condition, the fuel and oxidant flows from the fuel and oxidant sources 16 and 18, respectively, to the corresponding valve assemblies 22. Some of the valves assemblies 22 are controlled by the controller 752 to then switch to their open condition. In their open condition, the valve assemblies 22 direct the fuel and oxidant to the respective mixer tubes 736. The fuel and oxidant are directed along the flow paths 738 in the mixer tubes 736 and further into the plenum 710. The fuel and oxidant can commingle in the plenum 710.

[0072] During operation, the controller 752 responds to a predetermined trigger event, such as reaching a predetermined temperature. In response to the event, the controller 752 switches from operating in a particular mode to operating in a different mode. Specifically, the controller 752 shifts the valve assemblies 22 in accordance with the modes of operation described above. Different ones of the valve assemblies 22 can be shifted into or out of their open conditions in consecutive cycles.

[0073] The controller 752 switches the valves 756 to operate in the low or high temperature mode. In the low temperature mode of operation, the controller 752 directs the valves 756 to partially open to supply fuel and oxidant to the plenum 710 at the first flow rate. The first flow rate is sufficiently low so that if the fuel and oxidant is ignited in the plenum 710 the resulting combustion of a portion of the fuel and oxidant can occur while the portion is still within the plenum 710. Specifically, in the low temperature mode, the fuel and oxidant are ignited while the fuel and oxidant are still in the plenum 710 by an igniter, not shown, as known in the art. The ignition of the fuel and oxidant begins the production of hot combustion products. Because the first flow rate is sufficiently low, a portion of the combustion products are produced in the plenum 710 prior to the fuel and oxidant leaving the plenum. The resultant combustion products are directed by the plenum 710 through the array of burner ports 732 and into the reaction zone 14. Further combustion of the combustion products occurs in the reaction zone 14.

[0074] In the high temperature mode of operation the controller 752 directs the valves 756 to partially open to supply fuel and oxidant to the plenum 710 at the second, higher flow rate. The fuel and oxidant commingle in the plenum 710. Because the fuel and oxidant are traveling at the second, higher velocity, they subsequently exit the plenum 710 through the array of burner ports 732 prior to their ignition. Ignition of the fuel and oxidant occurs only in the reaction zone 14 in this mode of operation.

[0075] As noted above, combustion of the fuel and oxidant creates hot combustion products. These combustion products are recirculated in the reaction zone 14 by the pumping force caused by the fuel and oxidant flows 742 flowing through the array of burner ports 732. The recirculation flows 726 impinges on the fuel flows 722, thus entraining and urging them toward the burner axis 27.

[0076] Another differently configured furnace system may be used in accordance with the invention. An apparatus 800 comprising a seventh embodiment of the invention is shown in FIG. 17. The apparatus 800 is a furnace system and has parts that are substantially the same as corresponding parts of the burner 10. This is indicated by the use of the same reference numbers for such corresponding parts in FIGS. 1 and 17. For instance, the reaction zone 14 and the valve assemblies 22 are the same. However, the furnace system 800 differs from the furnace systems in previous embodiments in that it includes a different control system 802 and different first and second burner systems 804 and 806. Specifically, the first and second burner systems 804 and 806 differ from the burner systems of the other embodiments in that each has inner surfaces that define first and second oxidant plenums 810 and 812.

[0077] The first and second burner systems 804 and 806 are alike and communicate with the reaction zone 14 through a furnace wall 808 by means of pairs of first and second mixer tubes 820 and 822. Some of the first mixer tubes 820 communicate with the first plenum 810 of the first burner system 804 and other first mixer tubes communicate with the first plenum 810 of the second burner system 806. Similarly, some of the second mixer tubes 822 communicate with the second oxidant plenums 812 of the first burner system 804 and other of the second mixer tubes 822 communicate with second plenum 812 of the second burner system 806.

[0078] A first oxidant supply 824 supplies a first oxidant to the first plenums 810. A second oxidant supply 826 supplies a second oxidant to the second plenums 812. In this embodiment, the first and second oxidants can differ in composition and temperature. Specifically, the first oxidant can include flue gas supplied from a flue gas supply 828 while the second oxidant does not include flue gas. The addition of flue gas to the first oxidant dilutes the oxidant content and increases the temperature of the first oxidant.

[0079] A first fuel supply 830 supplies a first fuel to some of the valve assemblies 22. Those valve assemblies 22 supply the first fuel to the first mixer tubes 820 of the first and second burners 804 and 806. A second fuel supply 832 supplies a second fuel to other of the valve assemblies 22. These valve assemblies 22 supply the second fuel to the second mixer tubes 822.

[0080] As described above, the first and second mixer tubes 820 and 822 are supplied with oxidant from their respective plenums 810 and 812 and fuel from their respective valve assemblies 22. The mixer tubes 820 and 822 can mix their oxidant and fuel to create premix. The mixer tubes 820 and 822 can direct the premix into the reaction zone 14. Ignition of the premix can then occur in the reaction zone 14.

[0081] The control system 802 includes a controller 840 similar to the controllers 42, 226, 530 and 618 described above. Also, the control system 802 includes a plurality of valves 856. The valve assemblies 22 as well as the valves 856, are controlled by the controller 840.

[0082] Some of the valves 856 are located between the first fuel supply 830 and some of the valve assemblies 22. Other valves 856 are located between the second fuel supply 832 and other of the valve assemblies 22. Further, some of the valves 856 are located between the first oxidant supply 824 and the first plenums 810, while other of the valves 856 are located between the second oxidant supply 826 and the second oxidant plenums 812.

[0083] The valves 856 can have open, closed and partially open conditions. Flows of fuel and oxidant are interrupted if the respective valves 856 are in a closed condition and the flows are not interrupted if the respective valves 856 are in an open condition. The valves 856 restrict the respective flows if they are in the partially open condition. Further, the partially open condition includes a plurality of valve settings of differing degrees of restriction. The degrees of restriction correspond to different degrees of openness of the valves 856.

[0084] The controller 840 can control some of the valves 856 to open without opening other of the valves 856. By opening some of the valves 856, the first oxidant can be supplied from the first oxidant supply 824 to the first plenum 810. With additional other of the valves 856 open, the second oxidant is supplied from the second oxidant supply 826 to the second oxidant plenum 812. The valve assemblies 22 can draw oxidant from the respective plenum and direct it to the associated mixer tube. Because the valve assemblies 22 are controlled by the controller 840 and can draw oxidant from their respective plenums 810 or 812, the controller 840 can thus control the flow of oxidant to the respective mixer tubes 820 and 822.

[0085] Differences in oxidant content in the first and second oxidants can be controlled by the controller 840. Specifically, the controller 840 can control the valve 856 that is located between the flue gas supply 828 and the first oxidant supply 824. The controller 840 can control the valve 856 to open, close, or partially open. Thus, the amount of flue gas in the first oxidant can be controlled by the controller 840. The controller 840 can control the valves 856 located between the fuel supplies 830 and 832 to open, close, or partially open. Opening some the valves 856 can supply fuel from the first and second fuel supplies 830 and 832 to the respective valve assemblies 22.

[0086] The controller 840 can also operate in a mode in which it controls a first flow of oxidant from the first oxidant supply 824 to the first oxidant plenum 810 separately from a second flow of oxidant from the second oxidant supply 826 to the second oxidant plenum 812. Thus, the controller 840 can interrupt the first oxidant flow while continuing the second oxidant flow. That is, the controller 840 can shift a predetermined one of the valve assemblies 856 into and out of its open condition. The shifting can be done in consecutive cycles. During a cycle, a different one of the valve assemblies 856 can be shifted into or out of its open condition.

[0087] The controller 840 can also operate in a mode in which it controls some of the valves 856 to switch to a partially open condition that partially restricts the flow of the oxidant passing through the valve 856. Further, the controller 840 can operate in a mode in which it controls some of the valves 856, and thus the rate of supply of the first and second fuels from the fuel supplies 830 and 832 to the respective valve assemblies 22. Specifically, the controller 840 can shift the valves 856 into different partially open positions. The different partially open positions correspond to different flow rates of the first and second fuels. These amounts may differ in response to, for example, the selection of a particular turn-down rate. Because the valve assemblies 22 can supply fuel and oxidant to respective mixer tubes, the controller 840 can control the flow rate of the fuels and oxidants to the respective mixer tubes 820 and 822.

[0088] During operation, the controller 840 operates in a mode in which some of the valves 856 are opened. The controller 840 controls the valves 856 to supply the first and second plenums 810 and 812 with first and second oxidants, respectively. Additional valves 856 are opened by the controller 840 so that the valve assemblies 22 are supplied with first or second fuels, as appropriate. The valve assemblies 22 in turn supply fuel and oxidant to their corresponding mixer tubes 820 and 822.

[0089] The first and second fuels are mixed with the first and second oxidants in the first and second mixer tubes 820 and 822, respectively, to form premix. The premix formed in the first mixer tubes 820 is directed by the first mixer tubes 820 into the reaction zone 14. The premix formed by the second mixer tubes 822 is also directed by the second mixer tubes 822 into the reaction zone 14. The premix is ignited in the reaction zone 14 by an igniter, not shown, as known in the art.

[0090] The controller 840 shifts its mode of operation to the other modes of operation, as described above. Specifically, the controller 840 controls some of the valves 856 to shift to partially open positions to change the flow rate of the fuel and/or oxidant being supplied to the respective mixer tubes 820 or 822. Further, the controller 840 controls some of the valve assemblies 22 to shift them into and out of their open positions in consecutive cycles. Preferably, different ones of the valve assemblies 22 are shifted in each consecutive cycle.

[0091] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

1. An apparatus for heating a process chamber, said apparatus comprising: a furnace structure defining a reaction zone communicating with the process chamber; a burner structure defining a burner port and communicating with said reaction zone through said burner port, said burner structure including a plurality of mixer tubes, said mixer tubes being configured to mix fuel and oxidant and to define flow paths for the fuel and oxidant, each of said flow paths extending through a respective mixer tube; a valve assembly operative to interrupt a flow of fuel and oxidant along one of said flow paths, said valve assembly being shiftable between a closed condition interrupting a flow of fuel and oxidant along said flow path, and an open condition not interrupting said flow; and a controller operative to shift said valve assembly into or out of its open condition, said controller having a mode of operation comprising consecutive cycles in which said valve assembly is shifted into and out of its open condition.
 2. The apparatus as defined in claim 1, wherein said burner structure further defines a plenum, and said plenum defines a plenum flow path for fuel and oxidant extending from said plurality of mixer tubes to said burner port.
 3. The apparatus as defined in claim 2 further comprising a perforated plate configured to cooperate with said burner structure to restrict communication of said burner structure with said reaction zone through said burner port.
 4. The apparatus as defined in claim 2, wherein said controller has a low temperature mode of operation comprising combusting fuel and oxidant are in said plenum and a high temperature mode of operation comprising combusting the fuel and oxidant in said reaction zone, said burner structure being shiftable between said high temperature mode and said low temperature mode; said controller having a mode of operation further comprising shifting said burner structure into or out of its high temperature mode of operation.
 5. The apparatus as defined in claim 1, wherein said valve assembly is further shiftable to a partially open condition restricting a portion of said flow, and said controller has a further mode of operation comprising consecutive cycles in which said valve assembly is shifted into and out of its partially open condition.
 6. The apparatus as defined in claim 1, wherein one of said plurality of mixer tubes has a diameter that differs from the diameter of one other of said plurality of mixer tubes.
 7. An apparatus as defined in claim 1 further comprising a flow measuring device communicating with said burner structure, a pressure regulating device operative to provide and maintain a predetermined value of fuel pressure between said flow measuring device and said burner structure, and a valve operative to provide and maintain a predetermined value of fuel flow resistance between said flow measuring device and said burner structure.
 8. The apparatus as defined in claim 7, wherein said controller is further operative to select said value of fuel pressure with reference to an output of said flow measuring device, and to direct said pressure regulating device to provide and maintain said value of fuel pressure.
 9. The apparatus as defined in claim 7, wherein said controller is further operative to select said value of fuel flow resistance with reference to an output of said flow measuring device, and to direct said valve to provide and maintain said value of fuel flow resistance.
 10. A method of operating a premix burner apparatus for heating a process chamber, the apparatus including a burner structure defining a plenum, the burner structure further defining a port from the plenum to the process chamber, and the burner structure including mixer tubes defining a plurality of separate flow paths, each flow path configured to direct both fuel and oxidant to the plenum, said method comprising: controlling flows of fuel and oxidant into the plenum along at least one of said flow paths separately from flows of fuel and oxidant along at least one other of said flow paths into the plenum, said controlling step comprising restricting a flow of fuel and oxidant along one of said flow paths in consecutive cycles while fuel and oxidant continue to flow along a different one of said flow paths.
 11. The method as defined in claim 10, wherein a flow of fuel and oxidant is interrupted along a different one of said flow paths in each of said consecutive cycles.
 12. The method as defined in claim 10, wherein said controlling step further comprises modulating the ratio of fuel to oxidant along at least one of the flow paths separately from the ratio of fuel to oxidant along at least one other of the flow paths.
 13. The method as defined in claim 10, wherein said controlling step further comprises modulating the ratio of fuel to oxidant along at least one of the flow paths while not modulating the ratio of fuel to oxidant along at least one other of the flow paths.
 14. A method of operating a premix burner apparatus for heating a process chamber, the apparatus including a burner structure defining a plenum, the burner structure further defining a port from the plenum to the process chamber, and the burner structure including mixer tubes defining a plurality of separate flow paths, each flow path configured to direct both fuel and oxidant to the plenum, the apparatus further including a fuel supply system configured to direct fuel from a fuel source to the burner structure, and a controller operative to control the fuel supply system, said method comprising: measuring the fuel flow rate at a first location in the fuel supply system; selecting a value of fuel pressure with reference to said measured fuel flow rate; controlling said selected value of fuel pressure at a second location in the fuel supply system between said first location and the burner structure; selecting a value of fuel resistance with reference to said measured fuel flow rate; and controlling said selected value of fuel flow resistance at a third location in the fuel supply system between said first location and the burner structure.
 15. A method as defined in claim 14 further comprising: measuring an oxidant flow rate in an oxidant supply system configured to direct oxidant from an oxidant source to the burner structure; determining an actual fuel to oxidant ratio on the basis of said measured fuel flow rate and said measured oxidant flow rate; comparing said actual fuel to oxidant ratio with a specified fuel to oxidant ratio; and selecting said values of fuel pressure and fuel flow resistance with reference to said comparison of said actual fuel to oxidant ratio with said specified fuel to oxidant ratio.
 16. A premix burner apparatus for combusting premix in a reaction zone, comprising: a burner structure defining first and second oxidant plenums, said burner structure including a first mixer tube communicating with said first oxidant plenum and a second mixer tube communicating with said second oxidant plenum; an oxidant supply system including first and second oxidant sources, said first oxidant source communicating with said first oxidant plenum, and said second oxidant source communicating with said second oxidant plenum; and a control system operative to control flows of oxidant from said oxidant sources to said oxidant plenums, and to interrupt a first one of said oxidant flows while continuing a second one of said oxidant flows; said control system including valve assemblies shiftable between a closed condition interrupting said first one of said oxidant flows and an open condition not interrupting said first one of said oxidant flows; said control system further including a controller operative to shift said valve assemblies into and out of their closed conditions, said controller having a mode of operation comprising consecutive cycles in which said valve assemblies are shifted into or out of their closed conditions.
 17. The apparatus as defined in claim 16, wherein said burner structure is one of a plurality of burner structures; said controller having a second mode of operation comprising interrupting a flow of oxidant from said oxidant sources to said first oxidant plenum of each of said plurality of burner structures while continuing a flow of oxidant to said second oxidant plenum of each of said plurality of burner structures.
 18. The apparatus as defined in claim 16 further comprising a fuel supply system including first and second fuel sources, said first fuel source communicating with said first mixer tube, and said second fuel source communicating with said second mixer tube; said control system being further operative to control fuel flows from said fuel sources to said mixer tubes, and to interrupt a first one of said fuel flows while continuing a second one of said fuel flows; said control system further including fuel valve assemblies shiftable between a closed condition interrupting said first one of said fuel flows and an open condition not interrupting said first one of said fuel flows; said controller being further operative to shift said fuel valve assemblies into and out of their closed conditions, said controller having a second mode of operation comprising consecutive cycles in which different said fuel valve assemblies are shifted into or out of their closed conditions. 